The invention relates to a method of coating substrates, especially printed circuit boards, on both sides.
Printed circuit boards that are to be coated on both sides with a solder resist, photo-structurable dielectrics or other materials for the technique of sequentially laying-out the printed circuit board have a certain number of holes, some of which are to be covered by solder resist and others of which are to be kept clear for the connection of the circuit, depending upon the layout of the conductor arrangement. It is known to coat printed circuit boards by moving the printed circuit boards through a pouring curtain of the lacquer to be applied, the holes in the printed circuit board being only lightly skimmed over by the poured lacquer, so that good processing conditions are created for the formation of holes in the finished printed circuit board, because the covering over the holes can easily be removed.
In order that the holes that are not required for the conductor arrangement to be applied are lastingly closed, it is also known first to provide on one side of the printed circuit board, by poured application, a coating that only lightly skims over the holes in the printed circuit board with poured lacquer, whereupon that coating is dried in a drying step to such an extent that a screen print can be so applied to the coating that the holes that are not required are lastingly covered or filled up. After the screen printing step, the opposite side of the printed circuit board is coated by means of poured application. The overall result is a procedure having a plurality of individual steps which require a corresponding amount of apparatus.
The problem underlying the invention is to simplify the method of coating printed circuit boards without having any adverse effects on the coating.
That problem is substantially solved according to the invention by coating one side of the printed circuit board by means of a coating process, especially by screen printing, in such a manner that the holes that are not required for the circuit layout are lastingly covered over and the holes that are required are covered over only slightly or are left clear, whereupon the other side of the printed circuit board is coated by means of a coating process, especially by curtain pouring or spraying, so as to give only a light covering over the holes that are required for the circuit layout.
Because the screen printing technique is used for coating one side of the printed circuit board it is possible, together with the coating to be applied, lastingly to cover or fill up those holes which are not required for the circuit layout, while the holes that are required for the circuit layout can be covered over only lightly or can be kept clear. In the second step, the opposite side of the printed circuit board is uniformly coated, for example by the pouring curtain technique, so that in the course of further manufacture the holes that are required for the circuit layout can easily be exposed. In this way, the advantages of the screen printing process are combined with those of the pouring curtain technique for the coating of printed circuit boards, a simplification of the coating process being achieved by the omission of an intermediate step.
A further advantage is that the two sides of a printed circuit board can be coated one immediately after the other and can then be dried together in one step, thus achieving a further simplification of the process, with the advantage that the quality of the coating is increased by the simultaneous drying of both sides.